Controlled nickel sulfide precipitation using gaseous hydrogen sulfide

In the precipitation of metal sulfides from solution, either aqueous or gaseous sources of sulfide can be used. In this work, batch aqueous and gaseous sulfide experiments confirmed that no precipitation of Ni2+ was possible when only the H2S(aq) sulfide species was available for reaction. Gaseous sulfide precipitation of Ni2+ using H2S(g) was carried out in a sernibatch bubble reactor. Alkalinity was supplied in the form of NaOH to facilitate formation of bisulfide ions and thus precipitate NiS. When NaOH flow rates were an order of magnitude lower than H2S(g) flow rates, the precipitation was controlled at approximately pH 3.5. Dissolution of precipitates was observed and confirmed in batch experiments by monitoring nickel and sulfide concentrations with time at varying Ni:S molar ratios. The molar ratio of NaOH to H2S(g) at which efficient NiS precipitation occurred with no significant reagent wastage was found to be approximately 1.66:1 for the flow rates tested. Sulfide wastage occurred through either accumulation in the reaction solution or being lost as unabsorbed H2S(g). Below this molar ratio, precipitation of Ni(OH)(2) may occur since the hydroxide ion acts as the precipitating agent in the absence of sufficient sulfide.